Global positioning system tag system

ABSTRACT

A communications system for determining the position and velocity of an object using a plurality of GPS signals transmitted by a plurality of GPS sources includes an interrogator remote from the object and responsive to the plurality of GPS signals. The interrogator transmits an RF signal including GPS source information and at least one of frequency information and time and code phase information of at least one of the GPS signals. The system also includes a transponder positioned on the object and responsive to the RF signal and the plurality of GPS signals. The transponder tracks one of the plurality of GPS signals in response to the GPS source information and the frequency information and time and code phase information. The transponder generates a correlation snapshot and transmits the snapshot to the interrogator. The snapshot includes a sampled coarse acquisition (C/A) code between the one of the plurality of GPS signals and a replica of the one of the plurality of GPS signals generated by the transponder at regular offsets of some fraction of a C/A code chip. The interrogator processes the correlation snapshot to determine the position and velocity of the transponder.

RELATED APPLICATION

This application is a continuation of application Ser. No. 09/560,797,filed Apr. 28, 2000 which claims the benefit of provisional applicationserial No. 60/132,046 filed on Apr. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a position measuring device and moreparticularly to a system that uses Global Positioning System (GPS)satellites to determine the position of an object.

2. Description of Related Art

It is often desirable to obtain the position and velocity of an objectsuch as an unmanned vessel. Such objects or vessels include, forexample, towed barges, aircraft and automobiles. With respect to a towedbarge, the reasons for obtaining the position and velocity are clear—thesafety of the waterways depends on knowledge of the location, course andspeed of all vessels, manned or unmanned. To this end, the U.S. CoastGuard has addressed the need for situational awareness on the waterwaysthrough the Ports and Waterways Safety System (PAWSS), Vessel TrafficServices (VTS), and the Automated Identification System (AIS)transponder. Any AIS equipped vessel returns identification, location,course and speed data through the VTS to the Vessel Traffic Center (VTC)which displays the waterway traffic situation.

Through wireless DGPS receivers the position of barges can be reportedto any interrogator in range. The interrogator can be the towing vessel,another manned vessel, or the VTC itself. The towing vessel can maintainprecise knowledge of the position of its barges and relay that data tothe VTC. A portable, self powered wireless DGPS receiver may be placedon each barge to be tracked. The receiver must operate unattended, forat least the duration of a voyage and preferably much longer, evenindefinitely. The barge environment is low in vibration and dynamics,but the unit can be expected to be dropped and mishandled duringtransport. The RF environment will include VHF marine band activity,urban RF noise, and marine radar transmissions. Some blockage of GPSsignals below 30° elevation could be expected, but the higher elevationsatellites should be visible while the unit is operating on a barge.There will be multipath in the environment from the host barge, otherbarges and vessels, and shore facilities.

There are at least three current solutions available to provide a DGPStag for a remote unpowered object such as a barge. All would require abattery system connected to existing technology such as a complete DGPStransponder, a GPS pseudorange transponder, or an RF samplingtransponder.

A complete DGPS transponder consists of a GPS receiver and a DGPS beaconreceiver integrated with a RF modem at the tag. The GPS receiver and theBeacon receiver must remain on continuously while the tag is deployed,waiting to be interrogated. Once the unit is interrogated, it mustreturn a position report. This system for providing a DGPS tag for aremote unpowered object is undesirable in that all the equipment must bepowered continuously. Thus the system consumes a large amount of power.

A GPS pseudorange transponder consists of a GPS receiver integrated witha RF modem. No beacon corrections are collected at the transponder. Thereceiver must remain on to have pseudoranges available on demand. Whenthe unit is interrogated, it transmits the pseudoranges back to thetowing vessel. DGPS corrections are collected and stored at the towingvessel, and applied to the received pseudoranges. The barge positioncalculation is completed at the towing vessel. This system also requiresthat the equipment remain powered continuously in order to providemeasurements on demand.

A RF sampling transponder down converts and digitizes the L1 band signaland transmits the RF samples themselves. It does not need to remain onbetween interrogations. Processing the RF samples for code correlationsis done entirely at the interrogator. While this application has thelowest power requirements, it must transfer large amounts of data whichpresents its own troubles in terms of complexity and reliability.

What has been needed and heretofore unavailable is a system with verylow power requirements that is small and easy to deploy. The systemshould use a simple but reliable communication link and should minimizedata transmission to further conserve power and reduce the possibilityof transmission error.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides a systemfor, and a method of, determining the position and velocity of an objectusing a plurality of GPS signals transmitted by a plurality of GPSsources.

In a first aspect the invention is related to a communications systemfor determining at least one of position and velocity of an object. Thesystem includes an interrogator and a transponder. The interrogator isremote from the object and responsive to the plurality of GPS signals.The interrogator transmits an RF signal comprising GPS sourceinformation and at least one of frequency information and time and codephase information of at least one of the GPS signals. The transponder ispositioned on the object and is responsive to the RF signal transmittedby the interrogator and the plurality of GPS signals. The transpondertracks one of the plurality of GPS signals in response to the GPS sourceinformation and the at least one of frequency information and time andcode phase information. The transponder generates a correlation snapshotand transmits the snapshot to the interrogator. The snapshot comprises asampled coarse acquisition (C/A) code between the one of the pluralityof GPS signals and a replica of the one of the plurality of GPS signalsgenerated by the transponder at regular offsets of some fraction of aC/A code chip. The interrogator processes the correlation snapshot todetermine the position and velocity of the transponder.

In a detailed aspect of the invention, the GPS source informationcomprises a pseudorandom noise (PRN) code number and the frequencyinformation comprises at least one of a carrier frequency and a Doppleroffset frequency. In another detailed aspect, the frequency informationcomprises the carrier frequency of the RF signal transmitted by theinterrogator and the transponder comprises a local oscillator that isphase-locked, through a phase lock loop, to the carrier frequency. Inother detailed facets of the invention the time and code phaseinformation comprises an offset measurement in chips, the correlationsnapshot includes a set of fixed-point correlator sums and a range ofoffset in chips and the correlation snapshot is obtained as the set ofcorrelator outputs summed over an integration interval.

In a second aspect the invention involves a method for determining atleast one of position and velocity of an object using a plurality of GPSsignals transmitted by a plurality of GPS sources. The method includesthe step of positioning an interrogator remote from the object. Theinterrogator being responsive to the plurality of GPS signals fordetermining GPS source information and at least one of frequencyinformation and time and code phase information of at least one of theGPS signals. The method also includes the step of transmitting an RFsignal comprising the GPS source information and the at least one offrequency information and time and code phase information of at leastone of the GPS signals from the interrogator. Also included is the stepof positioning a transponder comprising a plurality of correlators onthe object. The transponder being responsive to the RF signal and theplurality of GPS signals for: tracking one of the plurality of GPSsignals in response to the GPS source information and the at least oneof frequency information and time and code phase information, generatinga correlation snapshot and transmitting the snapshot to theinterrogator. The snapshot comprises a sampled coarse acquisition (C/A)code between the one of the plurality of GPS signals and a replica ofthe one of the plurality of GPS signals generated by the transponder atregular offsets of some fraction of a C/A code chip. The method furtherincludes the step of processing the correlation snapshot to determine atleast one of position and velocity of the transponder.

In a detailed facet of the invention, the step of generating thecorrelation snapshot comprises the steps of obtaining a noncoherent sumof a plurality of integrations using the plurality of correlators spacedone chip apart; determining the approximate signal peak from thenoncoherent sum; prepositioning the correlators at a code phasepredicted from the signal peak and performing an integration withone-eighth chip correlator spacing to produce a plurality of correlatorsums. In more detailed aspects the noncoherent sum is obtained by two,one millisecond integrations using twelve correlators spaced one chipapart and the plurality of correlator sums is produced by a tenmillisecond integration.

Other features and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of the GPS tag system showing theRF links and an interrogator communicating with a plurality oftransponders;

FIG. 2 is a schematic block diagram of a transponder of the GPS systemof FIG. 1;

FIG. 3 is a timing diagram of a correlation snapshot with a single GPSdata bit; and

FIG. 4 is a graph of correlation loss vs. Doppler error for 1, 2, and 10ms integrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, in which like reference numerals are usedto designate like or corresponding elements among several figures, inFIG. 1 there is shown a functional block diagram of a GPS tag system 10in accordance with aspects of the present invention. The GPS tag system10 may be used in various applications in which the position andvelocity of an object 16 is desired to be known. The followingdescription of the invention focuses on the use of the GPS tag system 10as a means of tracking the position of a barge being towed by a towingvessel. The invention, however, is by no means limited to thisapplication.

Returning to FIG. 1, the GPS tag system 10 includes an interrogator 12located on a base object 22 and at least one transponder 14, alsoreferred to herein as a “tag”, placed on a remote object 16 where powermay or may not be available. The interrogator 12 includes a receivingantenna 18 for receiving GPS signals 20 transmitted by one or more GPSsources, such as satellites. The interrogator 22 further includes atransceiving antenna 24 for communicating with the transponders 14. Thetransponders 14 also include a receiver antenna 28 for receiving GPSsignals 20 and a transceiving antenna 30 for communicating with theinterrogator 12. The interrogator 12 and transponders 14 transmit andreceiver RF signals 26 back and forth. The content of these RF signals26 is described below.

All messages from the interrogator 12 to transponder 14 start with awake-up signal. This is an unmodulated carrier transmitted forapproximately 10 milliseconds, possibly at a higher power than the restof the message. The purpose of the wake-up signal is to alert all thetransponders 14 in range that a message will follow. This message wakesthe transponders 14 from their passive standby state. The remainder ofthe message is a sync pattern and an ID for the transponder 14 beingpolled. In the simplest embodiment, three interrogator-to-transpondermessages are defined. One message is the broadcast to all transponders14 to identify themselves, the second message is the acknowledge messageto terminate the transponders 14 identification messages and finallythere is the interrogation message 32, also referred to herein as a“correlation snapshot command message.” In the case of the correlationsnapshot command message 32, the sync pattern and ID are followed bypre-positioning data and a tracking signal.

The entire process of taking a correlator snapshot and returning thedata to the interrogator 12 takes on the order of 40 to 50 milliseconds.For a query rate of one Hz the transponder 14 is powered for only 50milliseconds per second. This low duty cycle contributes to thereduction of power used by the GPS tag system 10, which in turn, greatlyincreases battery life, especially over known systems which requirecontinuous power. The transponder's correlation snapshot command message32 provides all of the information required for the transponder 14 tofind the GPS signals and construct the correlation snapshots 34 fortransmittal back to the interrogator 12. The correlation snapshotcommand message 32 data includes a PRN code number, chip number, andDoppler offset for each GPS signal source to be received. Thecorrelation snapshot command message 32 is followed by a tracking signalto supply reference time and frequency information to the transponder14. The frequency information is provided by the incoming carrier. Thetiming information comes from the code applied to the track signal. Ashort code sequence after the pre-positioning information will indicatethe GPS data epoch. Each transponder 14 correlator channel should beginsignal acquisition on the commanded code chip a fixed time after thedata epoch occurs as shown in FIG. 3.

The GPS system 10 optimizes the division of processing between a tag 14on a remote object such as a barge 16 and the interrogator 12 on a baseobject such as a towing vessel 22, to minimize transponder power drain.The present invention takes advantage of the fact that no information isdisplayed or used at the transponders 14 and therefore limits thetransponder processing to collection of RF samples of GPS signals 20 andcorrelations against code replicas of the GPS signals.

As previously mentioned, the interrogator 12 transmits an RF signalcomprising an interrogation message 32 to the transponders 14. Inresponse, each of the transponders 14 on a barge 16 transmits an RFsignal comprising a correlation snapshot 34 to the interrogator 12.Using the information contained in the correlation snapshot 34, theinterrogator 12 performs the processing necessary to determine thepseudoranges and the navigational characteristics of the transponders 14to provide the position and velocity of the object 16 upon which thetransponder is placed.

With respect to a barge towing system, prior to beginning a voyage, atransponder 14 is placed on each barge 16 in a convoy. The transponders14 are preferably placed on the highest flat metal surface available.The transponders 14 are manually switched from off to on once deployedon the barge 16. Once switched on, the transponder 14 is in a standbymode. As explained below, the transponder 14 includes a passive standbycircuit that maintains the transponder in the standby mode until aninterrogation message 32 is received.

After deployment of the transponders 14, the operator turns on theinterrogator 12. Once powered up, the interrogator 12 begins acquiringGPS signals 20. The GPS tag system 10 also includes a user interface 36which communicates with the interrogator 12. Through the user interface36 an operator may select an IDENTIFY TAGS mode. When this mode isselected, the interrogator 12 sends a BPSK modulated identificationrequest signal (not shown) which is received by all of the transponders14. This request signal is separate and distinct from the interrogationmessage 32. Each transponder 14 transmits a response message (not shown)after an internally generated random delay. The transponder 14 responseis on the same frequency and includes a 20 bit ID code BPSK modulated onthe signal. The interrogator 12 acknowledges receipt of individualtransponder 14 responses. Each transponder 14 continues the cycle ofdelaying a random time and transmitting its ID until it is acknowledgedor reaches a timeout. Collisions of transponder response messages mayoccur, but the identification function still takes place because eachtransponder 14 continues to transmit until it is acknowledged by theinterrogator 12. Once the transponder response messages are received,the user interface 36 then displays a list of all the transponders 14 inthe convoy. The operator can assign custom names to any of the tags 14,by default they are marked with their five character hexadecimal IDcodes. No two transponders 14 have the same ID code.

In one embodiment of the invention, the operator selects tracking modeon the interface 36 and selects a polling rate for updating the positionof each transponder 14. The interrogator 12 begins a round robin pollingof the transponders 14 and obtains a set of six satellite correlationsnapshots 34 from each transponder. The interrogator 12 computes the tag14 positions and displays them on the user interface 36. The GPS tagsystem 10 may also provide an alarm radius which sounds an alarm whenany of the transponders 14 get more or less than a selected distancefrom the interrogator 12. The interrogator 12 may periodically send outthe identification request message to see if any other transponders 14are coming into range. As a first towing vessel navigates into awaterway occupied by a second towing vessel, transponders 14 from bargesbeing towed by a second towing vessel may be responding to theinterrogator 12 associated with the second towing vessel. Theinterrogator 12 associated with the first towing vessel may pick upthese messages and plot the locations of the transponder as well. Alarmscan be set by the operator for such conditions as new transponder 14detection, minimum or maximum barge-to-barge range violation, maximumbarge-to-towing range exceeding a limit, and missing barge responses.

At the destination, the barges 16 are tied up and the interrogator 12 isset to standby. The interrogator 12 GPS receiver remains on to support aquick start of the next voyage. Interrogation of the transponders 14stops. The transponders 14 drop back into their standby modeautomatically when not interrogated. As previously stated, when in thestandby mode the transponder consumes less than a milliwatt of power.

With reference to FIG. 2, the transponder 14 includes a power subsystemcomprising an antenna switch 42, passive standby circuit 46, powersupply control 48 and a solar powered charge controller 50. The antennaswitch 42 is shown in its normally closed position, with the receiver 30feeding the passive standby circuit 46. During periods of inactivity,the transponder 14 is essentially powered down with the receiver 30feeding the passive standby circuit 46. The passive standby circuit 46is a tuned filter that includes a precision diode detector, a low-passfilter, and a comparator that drive the gate signal of the MOSFET powersupply control 48. An RF signal at the resonant frequency causes abuild-up of the low-pass filter DC output voltage until it passes thethreshold and triggers the comparator to switch on the power supply. Theonly current drawn in the standby mode is the bias current for theprecision diode detector and the current drawn by the comparator in theoff state. In most embodiments this should total less than 1 milliwatt,and it may be possible to bring it down to the tens of microwatts rangewith the proper diode detector design. The operation of this circuitvaries with the band selected for GPS tag system communications. Lowsensitivity of the detection circuit may be compensated for byincreasing the power of the interrogation message 32 for the wake-upportion of the waveform only.

In another embodiment, in order to deduce the possibility of falsetriggering of the transponder 14, a more advanced passive standbycircuit that relies on the presence of two tones above the noisebackground before triggering can be used. The advantage to this approachis the reduction in the false alarm rate and subsequent power savings.In this case the circuit is implemented with a triplet of passive tunedfilter receivers. Two of these are used to detect continuous waves (CW)tones signals from the interrogator 12. The third is used to measure thenoise and interference in the band of interest. The three outputvoltages are combined in a pair of comparators which controls the gatevoltage of the power MOSFET. The third receiver channel performs thesame function as an automatic gain control (AGC) in an active receiver,increasing the dynamic range of the standby mode receiver.

The transponder 14 further includes an RF transceiver 52, a phase-lockedloop/voltage controlled oscillator (PLL/VCO) 40, correlators/codegenerators 54, an analog-to-digital converter (ADC) 56, a GPS RF-to-IFconverter (GPS RF/IF) 58 and a microcontroller 60. Upon power up of thetransponder 14, the antenna switch 42 switches to the closed positionand the RF transceiver 52 receive signals from the interrogator 12. Thissignal may comprise an identification request signal, as previouslydescribed, or an interrogation message 32, also referred to herein as a“correlation snapshot command message.” The PLLNVCO 40 is slaved to thecarrier frequency on the incoming correlation snapshot command message32 from the interrogator 12. This is done to provide the precisefrequency information for coherent integration of the incoming GPSsignals 20.

The transponder 14 squares the incoming BPSK signal to get the referencecarrier that the VCO 40 is slaved to. The squaring wipes off the BPSKmodulation. The PLL loop 40 locks onto the squared carrier while thepre-positioning information is being sent in the BPSK modulation. Theinterrogation message 32 includes at least a 20 millisecond trackingsignal after the pre-positioning information which the oscillatorremains locked to while a correlation snapshot is taking place.

The correlator 54 receives the carrier frequency from the PLLNVCO 40 andprepositioning information from the microcontroller 60. The correlator54 performs correlation functions on GPS signals received by the antenna28 and processed by the GPS RF/IF 58 and ADC 56. Correlation sums areprovided to the microcontroller 60 and forwarded to the RF transceiver52 for transmission, as a correlation snapshot, to the interrogator 12.

As previously mentioned, the transponder 14 (FIG. 1) is designed totransmit a correlation snapshot 34 back to the interrogator 12. Thecorrelation snapshot 34 is the sampled coarse acquisition (C/A) codecorrelation function between the received GPS signal 20 and a replicagenerated at the tag 14 at regular offsets of some fraction of a chipover the range of at least a full chip. The correlation snapshot 34 isobtained as the set of correlator outputs summed over an integrationinterval and transmitted as a set of fixed point values. The correlationsnapshot 34 is formed in response to an interrogation message 32 thatestablishes the code phase and frequency offset. A typical interrogationmessage 32 consists of the requested pseudorandom noise (PRN) codenumber, the code phase offset in chips and the Doppler frequency offsetof the GPS signal 20. The transponder 14 responds with a set offixed-point correlator sums and a range offset in chips. In oneembodiment the correlation snapshot 34 comprises twelve correlatoroutputs of four bits each with each output representing an offset of oneeighth of a chip. A GPS carrier lock is not obtained at the transponder14 because the transponder does not remain on long enough to track theGPS carrier. Instead, precise GPS signal carrier frequency informationis provided by the RF carrier from the interrogator 12, which allows forcoherent integration without a GPS carrier tracking loop. Thetransponder 14 bandwidth and correlator spacing are sized to insure codephase noise less than one meter at one sigma.

In one embodiment, the correlator 54 of the transponder 14 containsseventy-two correlators grouped into six channels. Each channel uses itstwelve correlators to take a correlation snapshot 34 of a single GPSsignal 20. The correlation snapshot 34 takes place in three steps. Withreference to FIG. 3, the first step S1 comprises a noncoherent sum oftwo, one millisecond integrations using the twelve correlators spacedone chip apart. The noncoherent sum is used to maintain a widerbandwidth than would be the case with a single coherent two millisecondintegration. This locates the GPS signal 20 if the code phaseprepositioning error is less than ±1758 meters and the carrier frequencyprepositioning error is less than ±375 Hz.

In the second step S2 the twelve correlators are all prepositioned atthe code phase predicted from the signal peak in step S1. Eachcorrelator is separated from the next by the predicted Doppler frequencyin 62.5 Hz intervals. A two millisecond integration is performed. Theoutput of these correlators is compared at the end of step S2, and theDoppler frequency is predicted to within 31.25 Hz worst case for stepS3. In step S3 the twelve correlators perform a ten millisecondintegration with one-eighth chip correlator spacing. The correlator sumsare normalized by the transponder microcontroller and sent back to theinterrogator as a set of twelve, four-bit values along with the chipoffset calculated in step S1.

The interrogator 12 analyzes the correlation snapshot 34 for multipathestimation and computation of the pseudorange. The narrow correlatorspacing substantially eliminates some of the close-in multipaths byestimating primary and delayed raysat the interrogator 12. It issignificant to note that the entire correlation snapshot 34 takes placewithin the space of one GPS data bit, or twenty milliseconds. Theinterrogator 12 signals the transponder 14 to start the correlationwithin a few milliseconds of the data bit boundary. Since the data bitis not decoding at the transponder 14, integrating across the data bitboundary is avoided to prevent potentially subtracting from thecorrelator sums. In an alternative embodiment, the transponder 14comprises only one channel with twelve correlators. In this embodimentthe transponder 14 is sequentially polled to obtain correlationinformation for each GPS signal 20 in view by the interrogator 12. Thishas the advantage of simplifying the transponder 14 design whileincreasing only slightly the processing required at the interrogator 12.

In another alternative embodiment, the transponder 14 reduces the twelvecorrelator values taken over the 10 ms samples to a chip offset byfitting a triangular correlation function to the data points whichrepresent the correlation curve thereby calculating the peak correlationposition. This value can then be converted to a fixed point number andreturned along with the initial whole chip offset calculated during stepone which was the noncoherent sum of two, one millisecond integrationsusing the twelve correlators spaced one chip apart which established thewhole chip offset. This embodiment has the advantage of returning evenless data to the interrogator 12 or ten bits for a one meter resolutionof the peak correlation position and 4 bits for the whole chip offsetfor a total of 14 bits of data per satellite or 84 bits for a sixchannel configuration.

In a preferred embodiment of the GPS tag system 10, the transponder 14returns twelve, four bit correlator outputs per satellite to theinterrogator 12. For six satellites, 288 bits are transmitted. This issufficient for one meter pseudorange accuracy and allows the GPS tagsystem 10 to use multipath mitigation techniques.

In order to perform this signal detection and pseudorange measurement ina single GPS data bit, the transponder 14 must have precise frequencyinformation. FIG. 4 shows the correlation loss for the one, two and tenmillisecond integration intervals versus carrier frequency error. TheDoppler pre-positioning data in the interrogation message 32 does notaccount for the frequency error in the transponder 14 clock, it isrelative to the 1575.42 MHz nominal L1 frequency. The transponder 14tracks the carrier frequency of the interrogation message 32 using thePLLNVCO 40 to set the frequency for the local code generator. Thisallows for precise frequency information to be transferred from theinterrogator oscillator (not shown) contained within the interrogator12. This in turn, allows the transponder 14 to get a ten millisecondcorrelation function measurement without local GPS carrier trackingloops or an expensive temperature compensated crystal oscillator (TCXO).

As shown in FIG. 4, to keep the correlation loss above −6 dB the initialfrequency error for the 1 ms integrations of the correlation snapshot 34must be less than ±375 Hz at L1, or ±0.238 ppm. This is achievable withthe phase locked loop tracking the incoming interrogation signal 32. Inthe second step of the correlation snapshot, with 62.5 Hz Dopplerspacing of the correlators, the system is able to determine the signalDoppler to within 31.25 Hz. Referring again to FIG. 4, this keeps thecorrelation loss above −2 dB for the ten millisecond integration.

When the interrogator 12 constructs an interrogation message 32 for thetransponder 14 it predicts the code phase that the transponder will see.A C/A code chip is approximately 293 m in length. The transponder 14samples the correlation function over twelve code chips in the summedone millisecond integration intervals. Twelve chips cover ±6 chips orabout ±1758 meters of slant range uncertainty. Given that theinterrogator 12 position is known from its own GPS receiver and that thetransponder 14 must be within approximately one kilometer to be withincommunications range of the interrogator 12, the system 10 can predictthe satellite slant range within ±1758 meters without any difficulty.Predicting the Doppler frequency observed by the transponder 14 on thebarge 16 is also possible because the difference in GPS signal Dopplerbetween the towing vessel 22 and the barge cannot be more than 30 m/s,or 0.1 ppm, which is well within the initial bandwidth of ±0.238 ppm.

Within the GPS tag system 10, the interrogator 12 acts as a central GPSreceiver, by maintaining track of all the visible GPS satellites andremotely operating a set of slave correlators, i.e., the transponders14. The interrogator 12 provides the precise phase and frequencyinformation required at the tags 14 via the interrogation message 32 andhas the only expensive oscillator in the system. The transponder 14correlators integrate coherently without a GPS carrier tracking loopbecause the same GPS signal 20 is being tracked by the interrogator 12,which is prepositioning the transponder 14 at the correct Doppleroffset. The transponders 14 sample the correlation function byintegrating and summing the correlator outputs over the integrationinterval at each of the correlator taps and returning the scaled sums.The interrogator 12 uses the sampled correlation function to find abest-fit for the multipath-distorted correlation peak. The interrogator12 can then determine the precise phase of the code in the transponder14 and thereby recover the pseudorange to the satellite.

In a preferred embodiment of the GPS tag system 10, a two bit sampler isused to sample the data before the correlator. The use of a two bitsampler is significant. Most GPS receivers use one bit samplers to avoidthe need for an automatic gain control (AGC) circuit. One bit samplersare prone to suffer from interference, particularly continuous wave (CW)interference. Every zero crossing of the CW interference flips the bit,thus “stealing” the A/D convertor. A two bit sampler with AGC control ofthe magnitude bit density provides a substantial margin against CWinterference.

The GPS tag system 10 uses a custom communications waveform to supportthe transponder 14 design. The waveform is a BPSK modulated carrier. Inone embodiment the communications link consists of a 19,200 bits persecond on a 40.68 MHz carrier which is an industrial, scientific,medical (ISM) band. The selection of a VHF carrier frequency is notmandatory and the system could operate in UHF on the 915 MHz ISM bandshould that be desired, but the 40.68 MHz selection is convenient forthe passive standby circuit design. A 100 mw transmission from thetransponder 14 is sufficient for a one kilometer range at the VHFfrequencies. In one embodiment the interrogator 12 is operated at 200 mwto reduce sensitivity requirements at the transponder 14.

Some additional applications for the GPS tag system 10 includepositioning transponders 14 on skiers to locate them if they becomelost, positioning transponders 14 on aircraft and ground vehiclestaxiing around airports, so controllers have better information forpreventing ground collisions, and positioning transponders 14 on golfcarts to allow golf courses to track progress of play.

While several particular forms of the invention have been illustratedand described, it will also be apparent that various modifications canbe made without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited except asby the appended claims.

What is claimed is:
 1. A communications system for determining theposition of an object using a plurality of GPS signals transmitted by aplurality of GPS sources, said system comprising: an interrogator remotefrom the object and responsive to the plurality of GPS signals, theinterrogator adapted to transmit an RF signal comprising GPS sourceinformation and at least one of frequency information and time and codephase information of at least one of the GPS signals; and a transpondercomprising a plurality of correlators positioned on the object andresponsive to the RF signal and the plurality of GPS signals and adaptedto track one of the plurality of GPS signals in response to the GPSsource information and the at least one of frequency information andtime and code phase information and generate a correlation snapshot byobtaining a noncoherent sum of a plurality of integrations using theplurality of correlators spaced one chip apart, determining theapproximate signal peak from the noncoherent sum, prepositioning thecorrelators at a code phase predicted from the signal peak, andperforming an integration with one-eighth chip correlator spacing toproduce a plurality of correlator sums, the snapshot comprising asampled coarse acquisition (C/A) code between the one of the pluralityof GPS signals and a replica of the one of the plurality of GPS signalsgenerated by the transponder at regular offsets of some fraction of aCIA code chip, and transmit the snapshot to the interrogator; whereinthe interrogator is further adaptedto determine a pseudorange associatedwith the transponder.
 2. The system of claim 1 wherein the GPS sourceinformation comprises a pseudorandom noise (PRN) code number and thefrequency information comprises at least one of a carrier frequency anda Doppler offset frequency.
 3. The system of claim 2 wherein thefrequency information comprises the carrier frequency of the RF signaltransmitted by the interrogator and the transponder comprises a localoscillator that is phase-locked, through a phase lock loop, to thecarrier frequency.
 4. The system of claim 1 wherein the time and codephase information comprises an offset measurement in chips.
 5. Thesystem of claim 1 wherein the time information comprises the time ofarrival of the RF signal at the transponder.
 6. The system of claim 1wherein the correlation snapshot includes a set of fixed-pointcorrelator sums and a range of offset in chips.
 7. The system of claim 6wherein the correlation snapshot is obtained as a set of correlatoroutputs summed over an integration interval.
 8. The system of claim 1wherein the transponder comprises a plurality of correlators groupedinto at least one channel, each channel responsible for generating acorrelation snapshot of a single GPS signal.
 9. The system of claim 8wherein the transponder comprises seventy-two correlators grouped intosix separate channels.
 10. The system of claims 8 wherein thetransponder comprises twelve correlators grouped into one channel. 11.The system of claim 10 further comprising a controller for sequentiallypolling the transponder to track a plurality of GPS signals to obtain acorrelation snapshot for each GPS source in view of the interrogator.12. A transponder associated with an object and for use in conjunctionwith an interrogator remote from the object to determine at least one ofposition and velocity of the object using a plurality of GPS signalstransmitted by a plurality of GPS sources and an RF signal comprisingGPS source information and at least one of frequency information andtime and code phase information of at least one of the GPS signals, theRF signal transmitted by the interrogator; said transponder comprising:a RF transceiver adapted to receive the RF signal from the interrogatorand subsequently transmit a correlation snapshot back to theinterrogator; an antenna adapted to receive the plurality of GPSsignals; and a plurality of correlators adapted to: track one of theplurality of GPS signals in response to the RF signal; and generate thecorrelation snapshot by obtaining a noncoherent sum of a plurality ofintegrations using the plurality of correlators spaced one chip apart,determining the approximate signal peak from the noncoherent sum,prepositioning the correlators at a code phase predicted from the signalpeak, and performing an integration with one-eighth chip correlatorspacing to produce a plurality of correlator sums, the snapshotcomprising a sampled coarse acquisition (C/A) code between the one ofthe plurality of GPS signals and a replica of the one of the pluralityof GPS signals generated at regular offsets of some fraction of a C/Acode chip.
 13. The transponder of claim 12 wherein the plurality ofcorrelators are grouped into at least one channel, each channelresponsible for generating a correlation snapshot of a single GPSsignal.
 14. The transponder of claim 13 wherein the plurality ofcorrelators comprises seventy-two correlators grouped into six separatechannels.
 15. The transponder of claim 13 wherein the plurality ofcorrelators comprises twelve correlators grouped into one channel.
 16. Asystem for determining at least one of position and velocity of anobject using a plurality of GPS signals transmitted by a plurality ofGPS sources, said system comprising: an interrogator remote from theobject, the interrogator responsive to the plurality of GPS signals andadapted to determine GPS source information and at least one offrequency information and time and code phase information of at leastone of the GPS signals and transmit an RF signal comprising the GPSsource information and the at least one of frequency information andtime and code phase information of at least one of the GPS signals; atransponder comprising a plurality of correlators positioned on theobject, the transponder responsive to the RF signal and the plurality ofGPS signals and adapted to: track one of the plurality of GPS signals inresponse to the GPS source information and the at least one of frequencyinformation and time and code phase information; and generate acorrelation snapshot by obtaining a noncoherent sum of a plurality ofintegrations using the plurality of correlators spaced one chip apart,determine the approximate signal peak from the noncoherent sum,preposition the correlators at a code phase predicted from the signalpeak, and perform an integration with one-eighth chip correlator spacingto produce a plurality of correlator sums, the snapshot comprising asampled coarse acquisition (C/A) code between the one of the pluralityof GPS signals and a replica of the one of the plurality of GPS signalsgenerated by the transponder at regular offsets of some fraction of aC/A code chip and transmit the snapshot to the interrogator; wherein theinterrogator is further adapted to process the correlation snapshot todetermine at least one of position and velocity of the transponder. 17.The system of claim 16 wherein the GPS source information comprises apseudorandom noise (PRN) code number and the frequency informationcomprises at least one of a carrier frequency and a Doppler offsetfrequency.
 18. The system of claim 16 wherein the time and code phaseinformation comprises an offset measurement in chips.
 19. The system ofclaim 16 wherein the time information comprises the time of arrival ofthe RF signal at the transponder.
 20. The system of claim 16 wherein thecorrelation snapshot includes a set of fixed-point correlator sums and arange of offset in chips.